1. Field of the Invention
The present invention is directed to a control system for selectively activating and deactivating an apparatus and, more specifically, to an emergency control system for deactivating an abrasive blast apparatus when an operator experiences emergency conditions.
2. Discussion of the Related Art
Various manually-operated devices benefit from the incorporation of emergency control systems therein that deactivate the device when an operator experiences emergency conditions. Maximum utility from the emergency control system is realized in a situation where feedback is desired when the operator becomes incapacitated (falls, faints, or the like) or otherwise becomes unable to properly operate the device. In many situations, the feedback is necessary to deactivate the device and/or to alert others of the situation. Devices in which emergency control systems have been incorporated include outboard and inboard marine engines, lawnmowers, pressurized abrasive blast systems, chainsaws, drill presses, ripsaws, and the like. Current federal regulations require emergency control systems on certain devices used in the workplace. In particular, Occupational Safety and Health Administration (OSHA) regulations require emergency control systems on all abrasive blast equipment used in the workplace.
For illustration, a prior art abrasive blast system 70 having an emergency control system 80 is shown in FIG. 1. Abrasive blast system 70 includes a compressed air supply 100, a blast media vessel 102, an air valve 104, a media metering valve 106, a blast hose 108, and a nozzle 109. In operation, compressed air supply 100 pressurizes blast media vessel 102. When air valve 104 and media metering valve 106 are opened, compressed air enters blast hose 108 to form a pressurized stream, and blast media is introduced to the pressurized stream by gravity to form a pressurized blast stream. The pressurized blast stream expands to atmospheric pressure through nozzle 109, thus hurling the blast media against a target at high velocity.
Emergency control system 80 includes a manual switch 110, supply lines 112, a control valve 114, and control lines 116. Control valve 114 is placed in fluid communication with air valve 104 and media metering valve 106 such that, when actuated, control valve 114 selectively allows or prevents compressed air from entering control lines 116 and actuating air valve 104 and media metering valve 106. In this manner, selective actuation of control valve 114 activates and deactivates abrasive blast system 70.
Conventional abrasive blast emergency control systems are either pneumatically or electrically operable. A pneumatic emergency control system includes a selectively open and closed path defined by switch 110, supply lines 112 (hoses), and control valve 114. In operation, an operator depresses switch 110 which, depending on the particular configuration employed, opens or closes the pneumatic path. When switch 110 is depressed, control valve 114 is placed in a position that activates abrasive blast system 70. When the operator releases switch 110, which is spring loaded, control valve 114 is placed in a position that deactivates abrasive blast system 7.
An electrical emergency control system includes a selectively open and closed electrical circuit defined by switch 110, supply lines 112 (wires), and control valve 114, which is a spring loaded electrical solenoid. In operation, the operator depresses switch 110 which closes the electrical circuit, energizes valve 114, and activates abrasive blast system 70. When released, switch 110 opens the circuit, which de-energizes valve 114 and deactivates abrasive blast system 70.
Both emergency control systems have advantages and disadvantages in particular applications. Pneumatic controls are dependable but, depending on the length of supply lines 112, have relatively slow response times. The response time in a typical abrasive blast system having over two-hundred feet of blast hose and four hundred feet of supply line can be several seconds. Furthermore, supply lines 112 in pneumatic systems are usually formed of rubber hoses which add undesirable bulk to the system. Electrical controls are less bulky than pneumatic controls. However electrical controls are prone to wear in a hostile blasting environment, and where certain explosive atmospheric conditions are present, exposed electrical switches are prohibited at a point of use.